For the evaporation of material in vacuum or at low gas pressure several methods are known. Especially, these methods are used widely for the coating of surfaces. For this purpose a vapor source, is located in a vacuum chamber and a substrate to be coated is located at some distance from the vapor source. The coatings are obtained by the deposition of vapor on the desired surface of a substrate. Well known are devices in which a material to be evaporated is placed in a refractory crucible. This can be heated up in different ways, e.g. by the Joule-effect or induction.
Another known method is based on the sputtering of cathodes.
For the evaporation of material by means of the impact of electrons, electron beam evaporators with hot filament cathodes are widely used. In such devices, the power density required for sufficient evaporation is obtained by accelerating and focussing electrons emitted from a hot filament onto a small surface area of the material to be evaporated. Frequently, this electron beam is forced on a curved path by the action of magnetic field. A disadvantage of this method is a rather low current (e.g. 1 A) and high energy- of electrons (e.g. 10 keV) as well. Due to these facts nearly no reaction between electrons and residual gas or material vapor occurs in the evaporator. However, a reaction of electrons with the residual gas or vapor (leading to dissociation, excitation, and ionisation) is highly desirable in order to obtain coatings of higher quality. Also, to prevent sparking in the electron gun and to prevent destruction of the hot filament cathode the gas pressure must be maintained below 10.sup.-4 mb.
To eliminate these disadvantages electron beam evaporators, based on hollow cathode discharges with hot and cold cathodes have been developed as disclosed by C. T. Wan, D. L. Chambers and D. C. Carmichael in "Journ. of Vacuum Science and Technology", Vol. 8, N. 8, p. VM99, 1971. In contrast to the electron beam evaporators described above these devices can be operated at pressures above 10.sup.-4 mb. However, only the hot hollow cathode provides a desired current voltage characteristic. There are still serious disadvantages which reside in that the discharge is hard to ignite, the cathode material is sputtered and contaminates the coatings, and an ambient working gas atmosphere is necessary to maintain the discharge. Though an ambient gas may useful to obtain reactive coatings, it reduces the quality the obtained coatings in many cases. Hollow cathode evaporator are not used in industrial practice. Another known method disclosed in U.S. Pat. No. 4,197,175 is based on the bombardment of an anode with electrons from a low-voltage arc discharge. This discharge is sustained between a hot filament cathode placed in a chamber which is separated from the evaporation chamber and communicates therewith through an aperture, and an anode placed in the evaporation chamber. The material to be evaporated is connected to the anode. During operation, gas is introduced continuously into the hot cathode chamber and the gas expands into the evaporation chamber through said aperture. By pumping continuously the gas out of the evaporation chamber a pressure difference between the two chambers is maintained. Due to this mechanism the gas pressure in the hot cathode chamber is sufficiently high to sustain the arc discharge (2.10.sup.-2 mb), while in the evaporation chamber the gas pressure is one order of magnitude lower. During operation the arc discharge burns through the mentioned aperture and is directed to the anode. In addition, the discharge constricted already by the aperture is focussed by the action of an adequate magnetic field onto a small spot of the anodic material to be evaporated. Typical values are power inputs of 5 to 8 kW onto an anode spot of about 0.2 cm.sup.2. Like in the case of the hollow cathode discharges a higher activation of the evaporated material is obtained by means of the low-voltage arc. On the other hand, both discharges are sustained by an ambient working gas. Without a residual gas, no operation of the discharges is possible. For this reason, these methods are preferably used for producing reactive coatings like TiN. In order to obtain pure coatings the presence of an ambient gas is a disadvantage in many cases.
Further known is a method making use of the cathode spot evaporation in vacuum arcs.: This known method is disclose in the British Patent 1,322,670. Vacuum arcs are discharges between two electrodes placed initially in a high vacuum. Normally, the discharge is sustained by the evaporation and ionisation of cathode material. Characteristic features of these discharges are the so-called cathode spots. These are, depending on the current, one or several bright and quickly moving spots on the relative cold surface of the cathode. Nearly the whole current of the discharge is concentrated in the cathode spots resulting in current densities between 10.sup.5 and 10.sup.7 A/cm.sup.2. This high power-concentration leads to an intense erosion of the cathode material. The eroded material consists of ionized metal vapor as well as numerous very small molten droplets. This mechanism is closely related to the sputtering of cathode material mentioned above. Though no ambient working gas is necessary to operate this discharge, this method has two serious disadvantages: (1) This discharge is very unstable and requires either very high currents or must be reignited very often as disclosed by A. S. Gilmour Jr. and D. L. Lockwood, "Proceedings of the IEEE", Vol. 60, No 8, p. 977, 1972 and by J. D. Cobine and G. A. Farrall in "Journ. of Appl Physics", Vol. 31, No. 12, p. 2296, 1960; and (2) The molten droplets leaving the cathode with a high speed result in pinholes covering the coated surfaces. Numerous literature publications can be found concerning the basic physics and the application of vacuum arcs. A comprehensive presentation can be found in "Vacuum arcs",Theory and Application, J. M. Laffety, Editor, John Willey a. Sons, 1980. The conventional vacuum arc is characterized by the mentioned cathode spots, i.e. bright and quickly moving spots on the relative cold surface of the cathode. In these spots the electrons and vapor to sustain the discharge are produced. Normally, this discharge is very unstable and extinguishes in fractions of seconds. Due to this property vacuum arcs have found their main application in high-current circuit-breakers. Anode phenomena leading to anodic evaporation are only known for very high currents above several thousand A (H. C. Miller, IEEE "Transaction on Plasma Science", Vol. PS-11, No 2, p. 76, 1983). It has the aim of all current technical efforts to avoid anodic evaporation, since this effect leads to a failure of a circuit breaker.